Solid compositions which generate hydrogen by combustion, comprising an alkali metal borohydride or alkaline earth metal borohydride and strontium nitrate Sr(NO3)2

ABSTRACT

The invention relates to a solid composition which can decompose with the generation of hydrogen according to a self-sustaining combustion reaction after initiation of this reaction by an appropriate heat source, this composition being characterized in that it comprises an alkali metal borohydride or alkaline earth metal borohydride and strontium nitrate Sr(NO 3 ) 2 .

[0001] The present invention relates to the field of generators ofhydrogen, a gas widely used as fuel or reducing agent in numerousdevices or industrial processes.

[0002] A more specific subject-matter of the invention is novel solidcompositions which can decompose with the generation of hydrogenaccording to a self-sustaining combustion reaction, and the use of thesecompositions for supplying hydrogen to proton exchange membrane fuelcells.

[0003] Numerous solid compositions which generate hydrogen by combustionare known, in particular for producing hydrogen intended to act as fuelin chemical lasers.

[0004] Patent U.S. Pat. No. 3,948,699 discloses solid compositions whichgenerate hydrogen by combustion composed of a mixture of alkali metalborohydride, for example sodium borohydride NaBH₄, with a metal oxide,for example iron oxide Fe₂O₃.

[0005] However, the yields by mass of hydrogen produced are low, lessthan 5%, expressed by weight of hydrogen obtained with respect to thetotal weight of the composition.

[0006] Patent U.S. Pat. No. 4,064,225 discloses other solid compositionswhich generate hydrogen by combustion composed of a mixture of alkalimetal borohydride, for example sodium borohydride, with ammoniumsulphate (NH₄)₂SO₄ or ammonium dichromate (NH₄)₂Cr₂O₇.

[0007] The yields by mass are slightly higher, of the order of 6%.

[0008] Such hydrogen yields, less than or in the vicinity of 5%, provein practice to be inadequate, in particular when miniaturization ofsystems is desired, for example when it is desired to replace thebatteries of portable electronic systems, such as telephones andcomputers, with miniature hydrogen fuel cells.

[0009] Patent U.S. Pat. No. 4,673,528 discloses a solidhydrogen-generating composition which can be in the form of pelletswhich can be used as fuels in chemical lasers. The hydrogen-generatingcompositions disclosed in this patent are thermally stable and comprise,as percentage by mass, between 5 and 15% of an oxidizing agent chosenfrom lithium nitrate LiNO₃ and potassium nitrate KNO₃, from 80 to 90% ofmagnesium borohydride diammoniate Mg(BH₄)₂. 2NH₃ and optionally from 2to 15% of a binder, such as polytetrafluoroethylene. The hydrogen yieldsobtained with this type of composition can reach 12.5%.

[0010] A person skilled in the art who wishes to replace the batteriesof portable electronic systems, such as telephones and computers, withminiature hydrogen fuel cells is continually preoccupied with trying tofind novel solid compositions which generate hydrogen by combustionwhich are completely stable and which provide better yields by mass ofhydrogen.

[0011] The present invention provides a solution to this problem.

[0012] A more specific subject-matter of the invention is novel solidcompositions which can decompose with the generation of hydrogenaccording to a self-sustaining combustion reaction after initiation ofthis reaction by an appropriate heat source, the said compositioncomprising an alkali metal borohydride or alkaline earth metalborohydride and, as oxidizing salt, strontium nitrate Sr(NO₃)₂.

[0013] It has been found, unexpectedly, that such compositions make itpossible to provide a yield by mass of hydrogen which can reach 12%depending on the nature and the relative proportions of theconstituents, which constitutes a particularly advantageous technicaland economic advance for the abovementioned reasons.

[0014] According to a preferred alternative form, the compositionsaccording to the invention are devoid of organic matter, that is to saythat they are composed solely of inorganic compounds.

[0015] In a particularly preferred way, they are composed essentially ofan alkali metal borohydride or alkaline earth metal borohydride and ofstrontium nitrate Sr(NO₃)₂, that is to say that these constituents arethe predominant ones by weight. It will be appreciated that the sum ofthe contents by weight of alkali metal borohydride or alkaline earthmetal borohydride and of strontium nitrate Sr(NO₃)₂ is greater than orequal to 75%, better still greater than or equal to 90% and even greaterthan or equal to 95%, with respect to the total weight of thecomposition.

[0016] Compositions composed solely of alkali metal borohydride oralkaline earth metal borohydride and of strontium nitrate Sr(NO₃)₂, thatis to say for which the sum of the contents by weight of the twoconstituents reaches 100%, are particularly preferred. The term“composed solely” should be understood as meaning that the compositionscan nevertheless include the impurities present in the crude or purifiedalkali metal borohydride or alkaline earth metal borohydride and in thecrude or purified Sr(NO₃)₂ salt used, or alternatively additives, suchas stabilizers, whether these products are commercially available or aresynthesized according to conventional methods.

[0017] When the compositions are not composed solely of alkali metalborohydride or alkaline earth metal borohydride and of strontium nitrateSr(NO₃)₂, they can, for example, also comprise other metal, alkali metalor alkaline earth metal borohydrides and/or a metal hydride and/or otheroxidizing inorganic salts, such as alkali metal nitrates, ammoniumsulphate, ammonium dichromate and iron oxides.

[0018] According to another preferred alternative form of the invention,the alkali metal borohydride is chosen from the group consisting oflithium borohydride, sodium borohydride and their mixtures.

[0019] According to another preferred alternative form of the invention,the alkaline earth metal borohydride used can be magnesium borohydrideMg(BH₄)₂, which has available a high level of hydrogen.

[0020] Generally and preferably, according to the invention, the ratioof content by weight of alkali metal borohydride or alkaline earth metalborohydride to content by weight of strontium nitrate is between 1 and10, better still between 2 and 10 and preferably between 4 and 10.

[0021] One aim of the invention is to obtain hydrogen-generatingcompositions having a good yield by mass of hydrogen per gram of solidcomposition and which are combusted at a temperature which issufficiently high for the reaction to be self sustaining and to preventthe solid composition from being extinguished and thus from beingentirely consumed. The relative proportions by weight between theoxidizing agent and the reducing agent should be established so as toachieve this aim.

[0022] According to another preferred alternative form, the solidcompositions according to the invention are provided in the form of acompact material having a specific form, for example and preferably inthe form of pellets or grains. The grains can have any shape, preferablyspherical, ovoid or cylindrical.

[0023] The pellets can also have any thickness and any peripheralgeometry, for example circular, elliptical, square or rectangular.

[0024] The thickness of the pellets may not be constant.

[0025] The solid compositions according to the invention can be obtainedby analogy with the described methods used to produce the abovementionedsolid compositions of the state of the art, for example by simple mixingof the constituents, milling and then mechanical homogenization. It isalso possible to mill the constituents before the mixing oralternatively to use constituents already in pulverulent form.

[0026] The compositions can also be obtained by granulation.

[0027] When, preferably, it is desired to obtain a solid compositionwhich is provided in the form of a compact material, the granular orpulverulent homogeneous mixture of the various constituents can, forexample, be agglomerated by compacting in a pressing container havingthe shape and the dimensions which are desired for the compact material.

[0028] It is also possible to obtain a compact material by dissolvingand/or suspending the constituents in a liquid medium. Afterhomogenizing and placing in a mould with the appropriate dimensionsdesired for the compact material, the liquid is removed, for example byevaporation, which makes it possible to obtain a compact material.

[0029] Another subject-matter of the present invention is a process forthe generation of hydrogen by self-sustaining combustion of a solidcomposition comprising an alkali metal borohydride or alkaline earthmetal borohydride and, as oxidizing salt, strontium nitrate Sr(NO₃)₂.

[0030] According to this process, a pulverulent or granular homogeneoussolid composition comprising an alkali metal borohydride or alkalineearth metal borohydride and strontium nitrate Sr(NO₃)₂ is first of allprepared.

[0031] This composition is subsequently agglomerated using appropriatemeans, for example those mentioned above, so as to form a compactmaterial, and then the compact material is placed in a combustionchamber which is purged under an inert gas or placed under vacuum.

[0032] When the dead volume (the volume remaining in the chamber afterthe compact material has been placed therein) is low, such a purge mayin practice be unnecessary.

[0033] The combustion of the compact material is then initiated using anappropriate heat source, which brings about the self-sustainingcombustion of the material with generation of hydrogen up to the end ofthe combustion.

[0034] The appropriate heat sources which make possible the initiationof the combustion by the “Joule” effect are well known to a personskilled in the art, in particular electrical initiators. The use of anickel-chromium ignition filament placed in contact or coated with thecomposition to be initiated, on which a sufficient voltage and asufficient current intensity (and therefore a sufficient power) areimposed, is entirely suitable. It is possible, for example, for a givenvoltage, to increase the intensity of the current until the combustionis initiated.

[0035] In some cases, to promote the ignition, a conventionalrelay-ignition powder well known to a person skilled in the art can bepositioned between the filament and the compact material. In this case,use will preferably be made of a relay-ignition powder of the samenature as the compact material according to the invention, that is tosay having the same constituents but with a markedly lower ratio ofcontent by weight of alkali metal borohydride or alkaline earth metalborohydride to content by weight of strontium nitrate Sr(NO₃)₂, forexample between 0.1 and 1.

[0036] Another subject-matter of the present invention is a pyrotechnichydrogen generator, intended to supply hydrogen to a proton exchangemembrane fuel cell, comprising an abovementioned solid compositionaccording to the invention.

[0037] Fuel cells operating with hydrogen, also known as proton exchangemembrane fuel cells, are well known to a person skilled in the art.

[0038] Such a fuel cell is composed essentially of 2 parts:

[0039] the core of the fuel cell, composed of one or moreelectrochemical cells mounted in series, which produces the electricalenergy;

[0040] the fuel, namely hydrogen, reservoir.

[0041] Attached to these two main parts are auxiliary systems relatingin particular to supplying hydrogen to the core of the fuel cell, todischarging the water produced or to cooling.

[0042] An electrochemical cell of the core of the fuel cell isrepresented diagrammatically in FIG. 1.

[0043] It is observed, in this FIG. 1, that an electro-chemical cell 1of the core of the fuel cell supplies electrical energy from twoelectrochemical reactions carried out on two electrodes 2 and 3generally composed of carbon and separated by a proton exchange membrane4 acting as electrolyte and generally composed of fluoropolymersimpregnated with water. Hydrogen H₂ is oxidized on the anode 2 in thepresence of a generally platinum-based catalyst, the hydrogen beingseparated into protons H⁺and electrons e⁻. The stream of protonsH⁺passes through the membrane 4, while the electrons e⁻, which are notable to pass through the membrane 4, are captured by a current collector5 connected to an external electrical circuit 6 to rejoin the cathode 3.On the other side of the membrane 4, at the cathode 3, the protons H⁺andthe electrons e⁻recombine with oxygen O₂, generally originating from thesurrounding air, to produce water H₂O.

[0044] The pyrotechnic hydrogen generators according to the inventionare essentially composed of one or more chambers in which a solidcomposition according to the invention, separate means for initiatingthe combustion of the composition in each of the chambers, means foractuating this initiation and means for transferring the hydrogenreleased in the chambers to the anode of a cell of the core of the fuelcell are placed.

[0045] Preferably, the overall amount of hydrogen capable of beingsupplied by the generator is released dis-continuously by a separateinitiation of the solid compositions present in the various chambers.The mass of solid composition in each chamber can be identical ordifferent from one chamber to another. The last alternative form makespossible release of hydrogen in an amount suited to a specific need.

[0046] The various chambers can emerge in a chamber for the expansion ofthe hydrogen released, this chamber being connected to the anodecompartment of a cell or having one of its walls at least partiallyformed by the anode.

[0047] Another subject-matter of the present invention is a protonexchange membrane fuel cell using hydrogen as fuel, comprising at leastone electrochemical cell and one abovementioned pyrotechnic hydrogengenerator according to the invention connected to the anode compartmentof the cell.

[0048] The following nonlimiting examples illustrate the invention andthe advantages which it provides.

Example 1

[0049] Solid composition composed of a mixture of NaBH₄ and of Sr(NO₃)₂in the relative proportions by weight 60/40 respectively

[0050] A mixture of 90 g of NaBH₄ and 60 g of Sr(NO₃)₂ is milled andthen homogenized.

[0051] A fraction of the pulverulent and homogeneous mixture thusobtained is subsequently introduced into and then compacted in thecompression die of a pelletizer having the desired pellet geometry,under a pressure of 10⁷ Pa (100 bar).

[0052] The circular pellet thus obtained, with a diameter of 5 mm and amass of 80 mg, is subsequently introduced into a combustion chamber witha volume of 10 cm³ and equipped with a pressure gauge, a temperatureprobe and a conventional ignition device comprising a nickel (80weight%)-chromium (20 weight%) filament. The pellet is brought intocontact with the filament and then the chamber is purged with an inertgas (nitrogen) under an absolute pressure of 10⁵ Pa (1 bar).

[0053] The filament is subsequently heated by the Joule effect untilinitiation of the combustion.

[0054] Once initiated, the combustion of the composition isself-sustaining and lasts approximately 3 s.

[0055] The combustion temperature, measured experimentally from themaximum pressure, varies between 480 and 600° K. approximately followingthe firings.

[0056] On completion of the combustion, the chamber is allowed to coolto ambient temperature and then the pressure in the chamber is recorded.

[0057] The measured increase in pressure and the analysis of the gasespresent after combustion by chromatography coupled to a massspectrometer makes it possible to calculate a yield by mass of hydrogenof 5.32 to 5.88%, expressed as grams of hydrogen released per gram ofsolid composition.

[0058] DTA analyses were carried out on this composition formed of 60/40sodium borohydride NaBH₄ and Sr(NO₃)₂ to determine whether it wasstable.

[0059] DTA (Differential Thermal Analysis) is a calorimetric measurementtest. It consists in heating the composition from 15 to 200° C., thetemperature being increased by 8° C. per minute. During this rise intemperature, this test consists in plotting the temperatures of theendothermic or exothermic peaks and in measuring the energy released orabsorbed. If no energy is released or absorbed, this means that thecomposition is stable and thus that the constituents of the compositionanalysed are entirely compatible.

[0060] The tests carried out on the composition of Example 1, 60/40NaBH₄/Sr(NO₃)₂, show that this composition is stable. This is because,during the DTA test, no energy was released or absorbed during the risein temperature.

[0061] Examples 2 to 8 are carried out in exactly the same way, only therelative proportions by weight between the two constituents beingmodified.

[0062] For each of Examples 1 to 7, the results obtained experimentally,on the one hand, and the results obtained by calculations, on the otherhand, are shown in Table 1 below. Two or three tests were carried outfor each of the examples but only the mean result appears. For some ofthe examples, no experiment has yet been carried out and therefore onlythe results obtained theoretically appear. TABLE 1 NaBH₄/ TheoreticalExperimental Sr(NO₃)₂ yield of yield of ratio by hydrogen hydrogen mass(%) (%) Example 1 65/35 6.93 4.55 Example 2 60/40 6.39 5.90 Example 355/45 5.86 5.75 Example 4 50/50 5.33 — Example 5 40/60 4.2 — Example 630/70 3.0 — Example 7 20/80 0.47 —

[0063] The theoretical yield of hydrogen decreases, of course, when theproportion of hydrogen-carrying reducing agent decreases. However, themeasured yield only corresponds to the theoretical yield forsufficiently high values of combustion temperatures.

Examples 8 to 14

[0064] Solid compositions composed of a mixture of LiBH₄ and Sr(NO₃₎ ₂in different relative proportions by weight.

[0065] In this example, sodium borohydride NaBH₄ is replaced by lithiumborohydride LiBH₄. The preparation is carried out in exactly the sameway as in Example 1 described above, that is to say with the productionof a pellet with the same mass. As in Examples 1 to 7 above, therelative proportions by weight between the two constituents are varied.

[0066] The theoretical results calculated for the various compositionsare displayed in Table 2 below: TABLE 2 LiBH₄/ Sr(NO₃)₂ Combustion Yieldof ratio by temperature hydrogen mass (° K) (%) Example 8 70/30 26511.20 Example 9 60/40 743 10.20 Example 10 50/50 1251 9.26 Example 1140/60 1491 7.40 Example 12 30/70 1680 5.53 Example 13 20/80 2048 3.58Example 14 10/90 2392 0.02

[0067] The tests carried out on the composition of Example 9, 60/40LiBH₄/Sr(NO₃)₂, show that this composition is not completely stable.This is because, during the DTA test, an energy absorption of 112 J/gramwas observed between 113 and 124° C.

Examples 15 to 22

[0068] In these examples, an alkali metal borohydride, such as NaBH₄ orLiBH₄, is no longer used but rather an alkaline earth metal borohydride,Mg(BH₄)₂, and strontium nitrate Sr(NO₃)₂ is still used as oxidizingagent. The preparation is carried out in the same way as in thepreceding examples with different relative proportions by weight betweenthe two constituents. The theoretical results obtained by calculationsare displayed in Table 3 below. TABLE 3 Mg(BH₄)₂/ Sr(NO₃)₂ CombustionYield of ratio by temperature hydrogen mass (° K) (%) Example 15 80/20571 11.95 Example 16 70/30 1079 10.45 Example 17 60/40 1789 8.93 Example18 50/50 1947 7.16 Example 19 40/60 2049 5.51 Example 20 30/70 2145 4.04Example 21 20/80 2673 1.74 Example 22 10/90 2477 0.01

[0069] Measurements were carried out with a pellet having the twoconstituents with relative proportions by weight of the Mg(BH₄)₂ to thestrontium nitrate Sr(NO₃)₂ of 70/30 respectively. A yield by mass of H₂,expressed as grams of hydrogen released per gram of solid composition,of 9.55% was obtained as results.

1. Solid composition which can decompose with the generation of hydrogenaccording to a self-sustaining combustion reaction after initiation ofthis reaction by an appropriate heat source, characterized in that thesaid composition comprises an alkali metal borohydride or alkaline earthmetal borohydride and strontium nitrate Sr(NO₃)₂.
 2. Solid compositionaccording to claim 1, characterized in that it is provided in the formof a compact material.
 3. Solid composition according to claim 2,characterized in that the compact material is a pellet or a grain. 4.Solid composition according to claim 1, characterized in that it isdevoid of organic matter.
 5. Solid composition according to claim 1,characterized in that it is composed essentially of alkali metalborohydride or alkaline earth metal borohydride and of strontium nitrateSr(NO₃)₂.
 6. Solid composition according to claim 1, characterized inthat the sum of the contents by weight of alkali metal borohydride oralkaline earth metal borohydride and of strontium nitrate Sr(NO₃)₂ isgreater than or equal to 90% with respect to the total weight of thecomposition.
 7. Solid composition according to claim 1, characterized inthat the alkali metal borohydride is chosen from the group consisting oflithium borohydride, sodium borohydride and their mixtures.
 8. Solidcomposition according to claim 1, characterized in that the alkalineearth metal borohydride is magnesium borohydride.
 9. Solid compositionaccording to claim 1, characterized in that the ratio of content byweight of alkali metal borohydride or alkaline earth metal borohydrideto content by weight of strontium nitrate Sr(NO₃)₂ is between 1 and 10.10. Process for the generation of hydrogen by self-sustaining combustionof a solid composition comprising an alkali metal borohydride oralkaline earth metal borohydride and strontium nitrate Sr(NO₃)₂,characterized in that: a pulverulent or granular homogeneous solidcomposition comprising an alkali metal borohydride or alkaline earthmetal borohydride and strontium nitrate Sr(NO₃)₂ is prepared, thiscomposition is subsequently agglomerated using appropriate means, so asto form a compact material, the compact material is placed in acombustion chamber, the combustion of the compact material is initiatedusing an appropriate heat source, which brings about the self-sustainingcombustion of the material with generation of hydrogen up to the end ofthe combustion.
 11. Hydrogen generator intended to supply hydrogen to aproton exchange membrane fuel cell, characterized in that this generatoris a pyrotechnic generator comprising a solid composition according toclaim
 1. 12. Proton exchange membrane fuel cell using hydrogen as fuel,comprising at least one electrochemical cell (1) and one hydrogengenerator connected to the anode compartment of the cell (1),characterized in that this hydrogen generator is a pyrotechnic generatoraccording to claim 11.